Method of preparing a fuel element of fissionable oxide and burnable poison



Jan. 2, 1968 R G. ROSE METHOD OF PREPARING A FUEL ELEMENT OF FISSIONABLE OXIDE AND BURNABLE POISON Filed DSC. 5, 1965 f3 JI :l1 B 4 1 Y43 Q4 FIG. I. |.I4 5

NO BORON f5 5 3.8w/O U235 |.OB- eoppm BORON l 235 3.8 w/o U :c LOB-- Q x NO BORON |.O4 3.2 w/o u235 L4 |02 eOppm BORON 3.2 w/o L12-35 LOO 0 5,OOO |O,OOO 15,0700

REAOTOR LIFE TIME MwD/ MTU INVENTOR Richard G. Rose BY MTM EY United States Patent O 3,361,857 METHOD F PREPARING A FUEL ELEMENT 0F FISSIONABLE OXIDE AND EURNABLE POISON Richard G. Rose, East McKeesport, Pa., assiguor to Westingliouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 3, 1965, Ser. No. 511,438 8 Claims. (Cl. 264-,5)

This invention relates to a fuel element for nuclear reactors and, more particularly, it pertains to a fuel element that incorporates a pellet having burnable poison homogeneously distributed therein.

As the nuclear reactor art has developed, a neutron absorber or poison material generally known as burnable poison has been adopted which is capable of absorbing neutrons while producing no new or additional neutrons or transforming into new poisons as a result of neutron absorption. A typical burnable poison is boron, which, on being irradiated by thermal neutrons undergoes the reaction Bl-l-nl-eLi'-l-Hei. The thermal neutron absorption cross section is reduced from 3840 to 0.033 barns in the reaction. The addition of a burnable poison to a reactor can be used to (l) extend the life of fuel elements having increased enrichment, (2) reduce the number of control rods required for a given core life, and (3) control power gene-ration to achieve a relatively uniform output over the life of the core.

The uses of a burnable poison at various locations in reactor cores including within the fuel pellets, coatings on the pellets and in cladding of the tubes, have met with mixed results. Prior attempts, however, to provide fuel pellets having a burnable poison dispersed uniformly in the fuel component have been relatively unsatisfactory. 1

Where a sintered uranium dioxide fuel pellet is used containing about 100 p.p.m. of boron, the fabrication of the pellets by conventional powder metallurgy techniques lhas resulted in unacceptable and variable losses of boron (as much as 50%) during the sintering cycle.

It has been found that conventional techniques for the fabrication of sintered uranium dioxide pellets containing about 1-00 p.p.m. of boron as a burnable poison additive may be modified in accordance with the present invention to minimize the loss of boron occasioned by the sintering cycle and thereby overcome most of the prior art diiculties.

Accordingly, it is an object of this invention to provide a process for producing a nuclear fuel element having selected proportions of boron homogeneously distributed therethrough.

It is another object of this invention to provide a process for producing sintered uranium dioxide fuel pellets containing boron in amounts substantially equal to the quantity of boron added prior to the sintering cycle.

Finally, it is an object of this invention to overcome the foregoing problems and desiderata in a simple and effective manner.

For a better understanding of the nature and objects of the invention, reference is made to the following detailed description and drawings, in which:

FIGUREG is a longitudinal sectional view of a fuel element having a plurality of sintered pellets of uranium dioxide containing a burnable poison homogeneously distributed therethrough;

FIG. 2 is a transverse sectional view along the line II-II of FIG. 1; and

FIG. 3 is a graph showing the effect of burnable poison on the life of a reactor using various enrichments of U2as 3,361,857 Patented Jan. 2, 1968 prelirninarily pre-sintered in vacuum at about 300 C.

after which it is further pre-sintered in hydrogen at about 700 C., following which it is fully sintered in hydrogen at about 1675 C.

As shown in FIG. l, a fuel rod or element is generally indicated at 1 and comprises an elongated cladding tube 2 having end closures 3 and 4 for providing an enclosed chamber 5 in which a plurality of fissionable fuel pellets 6 are disposed in end-to-end abutment. The tube 2 and pellets 6 are preferably circular and have a length of about twice their diameter. The pellet diameter is slightly less than that of the tube and forms a clearance space 7 therewith to accommodate any swelling of the pellet during use in a reactor.

As shown in FIGS. 1 and 2, each pellet 6 includes a sintered body portion of fssionable material 8 such as uranium dioxide (U02) and a selected amount of a burnable poison. In making the pellets, uranium dioxide, for instance, suitably enriched, as for example 3 to 5% in the form of a powder having a particle size of to mesh is compacted and subsequently sintered following the schedule described previously, to obtain a pellet density after sintering of approximately 90 to 96% of the theoretical density. Thereafter, each pellet is surface ground, for example, by centerless grinding to a cylindrical shape having a diameter in the range of from 0.2 to 0.5 inch. Each pellet is then Washed in water and normally dried by vacuum whereupon it is ready for insertion into the tube 2.

The body portion of the fuel pellets may be composed of a substantial proportion of a dioxide of one of the iissionable isotopes, such as U235, U233, Pu239, and Th232 or mixtures thereof, the balance being U238 or other uranium or thorium derivatives. Uranium oxide (U02) has given excellent results.

In addition to the fssionable material such as enriched U02, each pellet 6 contains a burnable poison, such as boron, cadmium, gadolinium, sarnarium, and europium, all of which have a high initial neutron capture cross section and which, upon the absorption of neutrons, result in elements of low neutron capture cross section.

The element boron is particularly satisfactory when added in predetermined quantities for obtaining the object of a more uniform distribution of power generation for a given quantity of issionable material. Boron may be added in the form of boron metal powder, boron carbide (B4G), or as zirconium diboride (ZrBz).

An additional ingredient is a temporary or transient organic binder and lubricant which is added to the blended mixture of the uranium dioxide and zirconium diboride. The binder and lubricant facilitates the cornpacting and cold pressing of the pellet prior to the presintering operations.

The amount of boron atoms in the sintered pellet as initially used may vary from 25 to 500 parts per million (p.p.m.) depending upon the amount of burnable poison desired. The range of normally desirable Weight percentages for each pellet ingredient are listed in Table I for each type of boron compound that may be used.

TABLE I.-EQUIVALENTS OF INITIAL ADDITIVES FOR VARIOUS BORON COMPOUNDS 4 EXAMPLE A homogeneous mixture of the ingredients of the fuel Bproriin Amounts ofinitiaiacidinvesmt.Percent) Pellet iS prepared by blending a mixture of uranium Sgleefsd dioxide and zirconium diboride in the form of powders (ppm) 134C U0, Binder 5 having a preferred composition of 97.988 weight percent uranium dioxide and 0.051 Weight percent zirco- 25 0.003 98.036 1.901 iiium diboride. To the mixture is added a binder-lubri- Zr pp lyethylene glycol (HOCH2) (CH2OCH2)X (CHgOH). 10 T e polyethylene glycol may be a commercially avail- 25 0.012 9s. 027 1.901 t 50o 0.250 97.783 1. 001 able lubricant sold under the trademark Carbowax 4000. The mixture of the binder and lubricant is added P d dB 2 0W ere Own to the blend of uranium dioxide and zirconium diboride 9 l5 The mixture of the uranium dioxide, zirconium dibo- The methou by Whoh the green behets are uresutered ride, and the binder with lubricant is compacted by lcold and sintered involves the following sequence: Pressmg Into a Cyhndflcl P61161- (1) Fre siuter iu Vacuum for about 2 hours at about The pellet is then subjected to a pre-sinter in a vacuum 250 C to 350 C at 300 C. for two hours to burn olf the binder and lubri- (2) Fre siuter iu hydrogen for 2 to 3 hours 2tt about 2O cant without affecting the uranium dioxide and zirconium 600 to 700 C. dlboflde- (3) Full sinter in hydrogen for about 4 hours at a After @301mg the Pe11e-t0 looffl temperature 1t 1S Subtemperature ranging from about l500 to 1800" C. Jecd t0 a Second Pre-Sintellng 111 hydl'ogen at 0D@ 3t- The sequence has the purpose of removing the binder lnosphre I'ssure at 700i C for.a penod of abou? two by decomposition and volatilizatiori preventing oxidation 01.1 or L e, Immos@ O removmg any, GCCSS Dygeu of the zirconium diboride. The three-step sintering pio- N hlch may exlst m the fof m of hypefsllchlofpetuc I lra' cedure avoids the problem of dw loss of boron during nium dioxide, thereby leaving the stoichiometric uranium Simering during the processing of uranium dioxide pellets dioxide with the Original content of zirconium diboride.

containing boron Thereafter, the pellet is sintered in hydrogen at one The compacted or cold pressed green pellet is p atmosphere pressure for about four hours at 1675D C. Hminarily pm sinred in a partial Vacuum of 5 L a whereby the compacted powders are sintered into a dense temperature ranging from 250 to 350 C. and preferably body hal/lg a denty of approxlmately 90 to 96% of at 300 C. for a period of one and one-half to three me theOfelCel density. hours and preferably at two hours for the purpose of The resulting sintered pellet 1s surface ground by cenremoving by decomposition, volatilization and combusterless grinding procedure to the precise diameter wheretion the organic binder Without affecting the basic mix- FPO 1t s ready for msemon 1m@ a Claddmg tube 2 of a ture of U02 and ZrBZ. wel element;

The binder-lubricant may be composed of one of a he 311mm d loyflde (U02) Preferably mcludes-en' series of commercially available organic binder examples n che-d uranium dlfmlde Such as 32% or 38% Uranium of which are polyvinyl alcohol and a lubricant such as dloxlda The requledlfomn Content H lay Vary from 25 a polyethylene glycol, one commercially available lubri- '50 500 P-PJP T he ad d110n 0f @-051 Wlfh PfCCD'f ZlrCO- Cant being designated as Carbowax 4000- por instance, nium diboride is equivalent to the addition of 100 ppm. an aqueous mixture of the polyvinyl alcohol and Carboboron t? h@ Smtefed Puet5- wax 4000 is added to the blended powders or uranium in a Slmlaf Way other Ummm?, Phltomum and thoflum dioxide and zirconium boude oxides, and other burnable poisons may be made into The hydrogen pre-sinter step is performed between Peuet5- 600 and 700 C. for 2 to 3 hours and preferably .ai about The fmodifnmw data for Zlfcoawm dlborgde-hydrosen, 700 C for 2 boum The purpose of thiS step is to adjust zirconium diboride-oxygen, and zirconium diboride-water the oxygedurauium ratio Whereby ahy excess iu oxygen, vapor reactions show that the zitconium diboride-hydrosuch as in hyperstoichiometric U02, combines with water gen feat10f1sare lhefmodynaffllcally UnffVOabe. The vapor leaving stoichiometric uranium oxide and thereby hefflodlfnamlcauy favorable ZlfCOnlUm dlboflde-OXS/gell prevents the oxidation of boron from zirconium diboride feaclon 1S in the subsequent sintering step in the higher tempera- ZYBH O2 )ZI,O2 B2O3 ture ranges. 2

Actual or fun Smtermg of the pellet. 1S Conducted m Less favorable reactions, though still thermodynamically hydrogen at one atmosphere pressure, in a temperature favorable are ranfiiig from 1500 to l800 C. for up to six hours with theapreferred temperature being l675 C. for four hours. ZrB2l8I2CZZf-l-22II3BO3-l-5H2 The sintering causes densication of the pellet resulting Z)FB2-lr6 2 r 2+ BO2 -l-5H2 in a sintered dense body from the compacted powders- Analyses of the remaining elements in the pellet after The following example is illustrative of the present each of the steps of the process are shown in the table invention: as follows:

TABLE 11.ANALYTICAL RESULTS [P.p.m. except for O/U ratio] Zircoriiulii Diboride B Zi1 C N O/U to +170 mesh) Addition Two analyses were taken for each step in the procedure, namely, the as pressed, Vacuum pre-sintered at 300 C., Vacuum pre-sintered at 300 C.l-hydrogen pre-sinter at 700 C., etc. Though variations in boron and zirconium between each pair of tests exist, the boron content remains substantially constant throughout the entire processes of pre-sintering and sintering. The rst specimen of each test is used for boron and zirconium analyses as well as the ratio of oxygen to uranium, while the second test of each analyses is used for boron and zirconium as well as the analyses of carbon and nitrogen. As indicated in Table II, the oxygen to uranium ratio diminishes as the successive sintering processes are performed.

Accordingly, it is evident that by adopting the threestep process of irst pre-sintering a pellet in a vacuum at 300 C. followed by pre-sintering in hydrogen at 700 C., and Ifinally sintering in hydrogen at about 1675 C. provides `a pellet having the desired boron content for use as a burnable poison in a fuel element pellet. The process of the present invention therefore, satisiies all prior procedures which did not include the hydrogen pre-sinterng step at 700 C. and as a result caused the burn-up of approximately 50% of the boron during the nal sintering procedure at the more elevated temperatures.

FIG. 3 shows the effect of additions of a burnable poison, namely, boron, upon the reactivity of a reactor, where the effective reactivity constant (Keff) is plotted against the reactor lifetime as measured by megawatt days/metric tons of uranium (MWD/MTU). Curves A and C show the reactivity of 3.8 weight percent (w/o) enriched U235 and 3.2 w/o uranium 235, respectively. Both curves A and C extend to very high levels of initial reactivity and decreases linearly during the reactor lifetime.

Curves B and D show the reactivity for U238 enriched with 3.8 w/o U235 and enriched with 3.2 w/o U235 respectively, Where boron in 60 p.p.m. is added. Manifestly, the addition of 60 p.p.m. boron to the enriched U235 greatly reduces the initial reactivity of the enriched iissionable material with only minor reduction in the reactor lifetime.

In other words, the addition of a burnable poison such as boron greatly reduces the total number (or volume) of control rods that must be provided in a reactor if the burnable poison is not added. The upper ends of the curves A and C are `a measure of the total amount of control rod material which must be added where boron is omitted. Accordingly, the use of burnable poison is significant because rst fuel loading of the cores can be greatly increased, leading to longer core life. Burnable poison can be introduced in sucient quantities to lower the initial reactivity and result in fewer control rods. Moreover, the rods are further out of the core during operation which leads to improved power distribution. An additional significance resulting from the use of burnable poison is that the poison can be placed in locations in the core which will electively flatten the curve of the power distribution throughout the core.

It is understood that the `above specification and drawings are exemplary of the technical and economically feasible methods for incorporating burnable poisons in a fuel element and thereby make possible the extension of the core life of a nuclear reactor by economic means.

What is claimed is:

1. The method for preparing a fuel element of precise dimension embodying a predetermined proportion of a burnable poison for use in a nuclear reactor comprising the steps of: mixing quantities of ssionable oxide material, a selected amount of a burnable poison comprising an inorganic compound, and an organic binder into a homogeneous blend, compressing the mixture into a compact mass, heating the compact mass in a Vacuum for 11/2 to 3 hours at a temperature ranging from about 250 to 350 C. to cause the organic binder to be removed by decomposition and volatilization, heating the compact mass in a hydrogen atmosphere for l to 3 hours at a temperature ranging from about 600 to 700 C. to remove excess oxygen, and sintering the compact mass in a hydrogen atmosphere for 1 to 6 hours at a temperature ranging from about 1500 to 1800" C.

2. The method of claim 1 in which the ssionable material is selected from a group consisting of uranium dioxide, plutonium dioxide, thorium dioxide, and any combination thereof, and in which the burnable poison is selected from a group consisting of boron, gadolinium, cadmium, samarium, europium, and compounds thereof.

3. The method of claim 1 in Which the compact mass is Vcooled to room temperature after each heating step.

4. The method of claim 1 in which the step of heating the compact mass in a vacuum atmosphere is performed at 300 C. for 2 hours.

5. The method of claim 1 in which the step of heating the compact mass in a hydrogen atmosphere is performed at 700 C. for 2 hours.

6. The method of claim 1 in which the step of sintering the compact mass in hydrogen is performed at 1675" C. for 4 hours.

7. The method of claim 1 in which the compact mass is heated in a vacuum at about 300 C. for 2 hours, and in which the compact mass is heated in hydrogen at 700 C. for 2 hours.

8. The method of claim 1 in which the compact mass is heated in a vacuum at about 300 C. for 2 hours, in which the compact mass is heated in hydrogen at 700 C. for 2 hours, and in which the compact mass is sintered in hydrogen at 1675 C. for 4 hours.

References Cited UNITED STATES PATENTS 3,051,566 8/1962 Schwartz 264-.5 3,230,280 l/l966 Kennedy 264.5 3,236,921 2/1966 Sermon 264-.5 3,263,004 7/1966 Bean 264-.5 3,320,176 5/ 1967 Davis 264-.5 XR 3,329,744 7/ 1967 Kaufmann et al. 264-.5

L. DEWAYNE RUTLEDGE, Primary Examiner. BENJAMIN R. PADGETT, Examiner. S. I. LECHERT, JR., Assistant Examiner, 

1. THE METHOD FOR PREPARING A FUEL ELEMEN OF PRECISE DIMENTION EMBODYING A PREDETERMINED PROPORTION OF A BURNABLE POISON FOR USE IN A NUCLEAR REACTOR COMPRISING THE STEPS OF: MIXING QUANTITIES OF FISSIONABLE OXIDE MATERIAL, A SELECTED AMOUNT OF A BURNABLE POISON COMPRISING AN INORGANIC COMPOUND, AND AN ORGANIC BINDER INTO A HOMOGENEOUS BLEND, COMPRESSING THE MIXTURE INTO A COMPACT MASS, HEATING THE COMPACT MASS IN A VACUUM FOR 1 1/2 TO 3 HOURS AT A TEMPERATURE RANGING FROM ABOUT 250* TO 350*C. TO CAUSE THE ORGANIC BINDER TO BE REMOVED BY DECOMPOSITION AND VOLATILIZATION, HEATING THE COMPACT MASS IN A HYDROGEN ATMOSPHERE FOR 1 TO 3 HOURS AT A TEMPERATURE RANGING FROM ABOUT 600* TO 700*C. TO REMOVE EXCESS OXYGEN, AND SINTERING THE COMPACT MASS IN A HYDROGEN ATMOSPHERE FOR 1 TO 6 HOURS AT A TEMPERATURE RANGING FROM ABOUT 1500* TO 1800*C. 